This is something I always wondered when it comes to Twitter and other services -- when a URL is displayed as "www.mywebsi..." and gets cut off with a "...", how is the link counted? As a full anchor text link, or a partial anchor text link? This isn't a question about nofollow/dofollow, just about syntax & anchor text in a cut off link.
I would imagine that if you're using the web interface and check the source, the full http:url will be listed and only the display is shortened. I haven't see that recently though as it seems that links are getting shortened whether they're too long or not. Unrelated to your question but shorteners that use 301 redirects are also counted as backlinks.